1. Field of the Invention
The present invention relates to an electric wrench driving system, particularly to an electric wrench driving system of simple structure and well-defined movement.
2. Description of Related Art
As shown in FIG. 2, a conventional electric wrench 1 comprises a motor 11; a catching system 2; and a shaft 13. The catching system 2 has a main body 21, a catching piece 22 on the main body 21, two catching arms 23, and a spring 24, which connects the two catching arms 23. A balancing rod 221 extends from the catching piece 22 into the main body 21 towards the motor 11. A transverse hole 222 is bored through the balancing rod 221, passed through by a positioning pin 223. The two catching arms 23 each have a lower end, which is mounted on the main body 21, swing upward in an arc-like shape along the main body 21 and have an upper end, into which a groove 231 and another groove 232 perpendicular thereto are cut. The grooves 231, 232 respectively accommodate the balancing rod 221 and the positioning pin 223, fixing the catching piece 22. The shaft 13 has a driven end which passes through the main body 21 and carries an insertion piece 131 for engaging with the catching piece 22.
FIG. 3 illustrates the engaging function of the conventional electric wrench shown in FIG. 2. When the motor 11 drives the main body 21 of the catching system 2 in a rotational movement up to a certain velocity, as indicated by the arrow in FIG. 3, the two catching arms 23 are driven outward by a centrifugal force and do not contact the positioning pin 223, allowing the catching piece 22 to sway freely, for driving the shaft 13 via the insertion piece 131. However, when the catching arms 23 depart from the positioning pin 223 on the catching piece 22, the catching piece will not necessarily sway as indicated in FIG. 3, but sometimes sway the other way, as shown in FIG. 4. In this state, the catching piece 22 is oriented as indicated by the dotted lines in FIG. 4 and is unable to drive the shaft 13. Accordingly, with a reverse rotational movement of the main body, the catching piece 22 is possibly oriented in a way unable to drive the shaft 13. Then a user has to turn off and on the motor 11 or to reverse the rotation thereof or to change the orientation of the catching piece 22 to allow the catching piece 22 to engage with the insertion piece 131 and to drive the shaft 13 again. Therefore, the conventional electric wrench 1 will drive the shaft 13, but only with the inconvenience described above.